Filter devices with bypass valves are known in a plurality of embodiments and sizes. Within the filter device is a hollow cylindrical filter element, which removes contaminants from contaminated liquid fed through an inlet in the filter device. The liquid is passed through the filter element generally from the outside inward. The filter mat of the filter element is supported along a support pipe provided with apertures. The filter liquid is then conveyed through an outlet out of the filter device for recycling for further use. The longer the filter element is in filtering operation, the more it is clogged by contaminants, until finally the filter element forms an almost impenetrable barrier for the flow of the fluid. The fluid can no longer reach the outlet end of the filter device. To avoid an interruption of the flow of the fluid, which can potentially result in an operational breakdown of an entire hydraulic system, a bypass valve is provided within the filter device. The bypass valve includes a spring-biased or pretensioned closing part. With increasing fluid pressure within the filter device, the closing parts opens a fluid-conveying passage directly from the inlet to the outlet, disconnecting the filter element. Such bypass valves consequently support operational reliability and avoid undesirable pressure peaks, which can also lead to damage within a hydraulic system.
Such a filter device with bypass valve is known from U.S. Pat. No. 4,279,746. This filter device is provided with a bypass valve. When the filter becomes clogged, it frees a fluid-conveying passage to an outlet. A closing part is present, acted on by a breech-closing spring. An adjusting device is provided for adjusting the spring pretensioning, and thus, the opening force for the closing part. As adjusting device, this patent employs an adjusting ring which can be moved along a holder rod for the step-by-step adjustment of the spring pretensioning, and can be fitted and locked with this member in predeterminable catch settings. A clip-spring servces as a catch which can be inserted in the catch openings of the holder rod and represents an end stop for the pretensioned adjusting ring. With this arrangement, the maintenance for the adjusting of the opening force for the closing part is difficult and complicated to perform.
DE-A-4 214 500 discloses a holding cage in the form of a support body, configured as hollow cylinder with notch cutouts. Within this holding cage is arranged a pretensioned closing part, which opens when a predeterminable desired pressure peak is reached, so that an ambient or high-pressure function is realized. Modification of the spring closing force and with that of the opening force for the closing part is not possible for all practical purposes following the assembly of the device for successful control of the ambient or high-pressure valve, since the adjusting ring is fitted and locked with its detent in the holder cage.